Recombinant Pseudomonas exotoxins containing deletions in domain Ia of the native toxin and which exhibit low side effects are described in U.S. Pat. No. 4,892,827. However, the role of individual amino acids, either alone or in combination with other amino acid sequences in various domains of the PE molecule were not known. It has been shown, however, that domain Ia is required for the binding of the PE molecule to the target cells.